Like this:

The International Fixed calendar (also known as the Cotsworth plan, the Eastman plan, the 13 Month calendar or theEqual Month calendar) is a solar calendar proposal for calendar reform designed by Moses B. Cotsworth, who presented it in 1902.[1] It provides for a year of 13 months of 28 days each, with one or two days a year belonging to no month or week. It is therefore a perennial calendar, with every date fixed always on the same weekday. Though it was never officially adopted in any country, it was the official calendar of the Eastman Kodak Company from 1928 to 1989.

Rules

The calendar year has 13 months with 28 days each, divided into exactly 4 weeks (13 × 28 = 364). An extra day added as a holiday at the end of the year, sometimes called “Year Day”, does not belong to any week and brings the total to 365 days. Each year coincides with the corresponding Gregorian year, so January 1 in the Cotsworth calendar always falls on Gregorian January 1.[3] Twelve months are named and ordered the same as those of the Gregorian calendar, except that the extra month is inserted between June and July, and called Sol. Situated in mid-summer (from the point of view of its Northern Hemisphere authors), the name of the new month was chosen in homage to the sun.[4]

Leap year in the International Fixed Calendar contains 366 days, and its occurrence follows the Gregorian rule. There is a leap year in every year whose number is divisible by 4, but not if the year number is divisible by 100, unless it is also divisible by 400. So although the year 2000 was a leap year, the years 1700, 1800, and 1900 were common years. The International Fixed Calendar inserts the extra day in leap year after Saturday June 28 and before Sunday Sol 1.

Each month begins on a Sunday, and ends on a Saturday; consequently, every year begins on Sunday. All the months look like this:

Sun

Mon

Tue

Wed

Thu

Fri

Sat

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

Year Day and Leap Day are not considered to be part of any month.

The following shows how the 13 months and extra days of the International Fixed Calendar occur in relation to the dates of the Gregorian calendar:

Month

Starts

Ends

January

January 1

January 28

February

January 29

February 25

March

February 26

March 25*

April

March 26*

April 22*

May

April 23*

May 20*

June

May 21*

June 17*

Leap Day

June 17

Sol

June 18

July 15

July

July 16

August 12

August

August 13

September 9

September

September 10

October 7

October

October 8

November 4

November

November 5

December 2

December

December 3

December 30

Year Day

December 31

*These dates are a day earlier in a leap year.

History

The simple idea of a 13-month perennial calendar has been around since at least the middle of the 18th century. Versions of the idea differ mainly on how the months are named, and the treatment of the extra day in leap year.

The Georgian calendar was proposed in 1745 by an American Colonist from Maryland writing under the pen name, Hirossa Ap-Iccim (=Rev. Hugh Jones).[5] The author named the plan, and the thirteenth month, after King George II of Great Britain. The 365th day each year was to be set aside as Christmas. The treatment of leap year varied from the Gregorian rule, however; and the year would begin closer to the winter solstice. In a later version of the plan, published in 1753, the 13 months were all renamed for Christian saints.

Whether Moses Cotsworth was familiar with the 13-month plans that preceded his International Fixed Calendar is not known. He did follow Ap-Iccim (Jones) in designating the 365th day of the year as Christmas. His suggestion was that this last day of the year should be designated a Sunday, and hence, because the following day would be New Year’s Day and a Sunday also, he called it a Double Sunday.[6] Since Cotsworth’s goal was a simplified, more “rational” calendar for business and industry, he would carry over all the features of the Gregorian calendar consistent with this goal, including the traditional month names, the week beginning on Sunday, and the Gregorian leap-year rule.

Advantages

The several advantages of The International Fixed Calendar are mainly related to its organization.

Every year has exactly 52 weeks divided in 13 months.

Each month has exactly 28 days divided in 4 weeks.

The calendar is the same every year (perennial), unlike the annual Gregorian calendar, which differs from year to year. Hence, scheduling is easier for institutions and industries with extended production cycles.

Every day of the month falls on the same weekday in each month—the 17th always falls on a Tuesday, for example.

Statistical comparisons by months are more accurate, since all months contain exactly the same number of business days and weekends; likewise for comparisons by 13-week quarters.

Thirteen equal divisions of the year are superior to twelve unequal divisions in terms of monthly cash flow in the economy, or so supporters of the IFC argued.[10]

Disadvantages

For the superstitious, a disadvantage to this format is that every month includes a Friday the 13th, and this date occurs thirteen times every year. This is readily solved by making the 1st a Monday.

Thirteen, being prime, is not evenly divisible, putting all activities currently done on a quarterly basis out of alignment with the months; each quarter would be 13 weeks instead.

Some Jewish, Christian, and Islamic groups have been historically opposed to the calendar because their tradition of worshiping every seventh day would result in either the day of the week of worship changing from year to year or eight days passing when Year Day or Leap Day occurs.[11] Others have contended that Year Day and Leap Day could be counted as additional days of worship.